The natural hemopoietic chimerism in the marmoset, a member of the New World primates, provides a potentially unique tool for the study of mechanisms related to immunologic tolerance. This tolerance has been demonstrated by the complete acceptance of skin grafts exchanged between chimeric co-twin pairs and also by the lack of stimulation between lymphocytes from these pairs in mixed lymphocyte culture (MLC) reactions. The MLC test has also shown to be capable of distinguishing among different species and subspecies of marmosets, the responses obtained reflecting the probable immunogenetic relationship of these animals based on current taxonomic classification. Isoimmunization among these animals (in certain genetic combinations) with white blood cells and platelets has produced a disease process similar to post-transfusion purpura in man, a disorder falling within the clinical syndrome of idiopathic thrombocytopenic purpura (ITP). In addition to these studies concerted efforts are being made on (A) the procurement of non-chimeric marmosets, (B) the immune competence of the marmoset to a variety of antigenic stimuli, (C) the utilization of the MLC test to identify specific serum blocking factors, and (D) the identification of T and B lymphocytes.